1. Field of the Invention
The present invention relates to the field of three-dimensional ultrasound imaging. More specifically, the present invention relates to an ultrasound probe mounting assembly.
2. Description of the Prior Art
Three-dimensional (3D) ultrasound imaging is a technique in which a set of spatially related two dimensional ultrasound slices (tomograms) of a target are collected and mathematically converted to create a virtual ultrasound volume. This virtual ultrasound volume facilitates the visualization of non-acquired slices of the target and a variety of rendered surfaces and projections of the target otherwise unobtainable using two-dimensional (2D) ultrasound imaging.
High fidelity 3D ultrasound requires, by definition, a data set in which the spacial relationship between the individual ultrasound slices is precisely known. High fidelity ultrasound is important for the accurate assessment of volumes and the appreciation of target geometry. The conventional method of choice for obtaining the precise spatial relationship between ultrasound slices is to actively constrain the position of each ultrasound slice. This is achieved by controlling the position of the ultrasound probe during generation of the slices by use of a motorized positioning device (mechanical scanning). Examples of 3D ultrasound imaging systems are described in detail in commonly assigned U.S. Pat. Nos. 5,454,371 (Fenster et al.) and 5,562,095 (Downey et al.), the contents of each of which are hereby incorporated by reference.
Although the mechanical scanning approach to 3D ultrasonography offers speed and accuracy, the bulkiness of the devices at times hinders the scan, particularly when imaging large structures. To overcome this problem, investigators have developed various xe2x80x9cfree-handxe2x80x9d acquisition techniques in which the operator can hold an assembly, composed of the transducer and an attachment (as will be discussed below, and manipulate the assembly over the subject to be imaged. A computer records the conventional 2D images generated by the ultrasound machine as well as their position and angulation. Because the geometric information about the transducer""s location is not predefined, the exact relative position and angulation of the ultrasound transducer must be accurately known for each acquired image slice. This information is then used in the reconstruction of the 3D image in a manner that avoids distortions. Over the past two decades a number of free-hand scanning approaches have been developed which make use of three basic positioning techniques: acoustic; articulated arm; and magnetic field.
In acoustic position sensing, three sound emitting devices, such as spark gaps, are mounted on the transducer and an array of fixed position microphones are mounted above the patient. During scanning, the microphones continuously receive sound pulses from the transducer. The position and orientation of the transducer as each 2D image is acquired is determined by knowledge of the speed of sound in air and the time of flight of the sound pulses to the fixed microphones. This technique has a number of disadvantages, for example, the microphones must be placed over the patient in a way that provides unobstructed xe2x80x9clines-of-sightxe2x80x9d to the sound emitters and sufficiently close to allow detection of the sound pulses. Further, the speed of sound varies with temperature and humidity and so, in a given environment, corrections must be made to avoid distortions in the 3D image.
One of the simplest approaches to free-hand scanning is to mount the transducer on a mechanical arm system with multiple moveable joints. Potentiometers located at the joints of the arms provide information about the relative movement of the arm during scanning. This system also has a number of disadvantages. For example, to avoid distortion in the final image, the potentiometers must be accurate and precise and the arm system must not flex. Sufficient accuracy may be achieved by keeping individual arms as short as possible and reducing the number of degrees of freedom. However, increased precision is achieved at the expense of flexibility in scanning and the size of the volume that can be imaged.
Magnetic position sensing makes use of a six degree-of-freedom magnetic field sensor to measure the ultrasound transducer""s position and orientation. The approach makes use of a transmitter, which produces a spatially varying magnetic field, and a small receiver containing three orthogonal coils to sense the magnetic field strength. Although magnetic field sensors allow for less constrained geometrical tracking of the transducer, they are susceptible to noise and errors. For example, the devices are sensitive to electromagnetic interference form sources such as CRT monitors, AC power cables and ultrasound transducers.
It is an object of the present invention to provide an ultrasound transducer mounting assembly which allows for the determination of the spacial relationship between a succession of 2D image slices, which obviates and mitigates at least one of the disadvantages of the prior art.
Accordingly, in one of its aspects, the present invention provides a mounting assembly, for use with an ultrasound transducer attached to an ultrasound machine, for determining the spacial relationship between a succession of 2D image slices of a target of a subject, generated by the ultrasound machine, the mounting assembly comprising:
(i) means to mount an ultrasound transducer;
(ii) means to engage a surface of the subject in the proximity of the target, the means to engage a surface being moveably attached to the means to mount an ultrasound transducer; and
(iii) sensing means, in communication with the means to engage a surface, to measure the movement of the means to engage a surface during acquisition of the succession of 2D image slices.
In another aspect, the present invention provides an ultrasound transducer assembly, for use with an ultrasound machine, for determining the spacial relationship between a succession of 2D image slices of a target of a subject, generated by the ultrasound machine, the ultrasound transducer assembly comprising:
(i) an ultrasound transducer;
(ii) assembly means mounted to the ultrasound transducer;
(iii) means to engage a surface of the subject in the proximity of the target, the means to engage a surface being moveably attached to the assembly means; and
(iv) sensing means, in communication with the means to engage a surface, to measure the movement of the means to engage a surface during acquisition of the succession of 2D image slices.
Typically, the target to be scanned will be beneath the surface of the subject. Thus, the target may be an internal organ and the mounting assembly is translated over the skin of the patient. However, the present invention should not be limited in that sense.
Further, the xe2x80x9cmeans to engage the surface of the subjectxe2x80x9d is intended to have a broad meaning in this specification. Specifically, as will be developed in more detail hereinbelow, this element can directly (e.g. wheel, roller, trackball,surface engaging tilt sensor and the like) or indirectly (e.g. non-surface engaging tilt sensor) engage the surface of the subject. Thus, the term xe2x80x9cengagexe2x80x9d is used to indicate that, in a notional sense, the element engages the surface of the subject to facilitate measurement of movement of the element by the sensing means.